As qualities of printing paper, concealing property of paper itself (opacity as white paper) and less strike-through of ink after printing (opacity after printing) are generally required. These qualities are conventionally improved by using amorphous silica as a filler. However, for example, amorphous silica is expensive, and since ratio of used paper contained in raw material of printing paper, especially newspaper, is rapidly increasing in recent years, the papermaking process of newspaper is going to be made neutral process. Therefore, there has been desired a filler usable in a neutral environment, which can replace amorphous silica and improve opacity as white paper and opacity after printing.
Meanwhile, calcium carbonate is widely used as pigment, filler etc., and light calcium carbonate of which properties are improved for use as a filler for papermaking has also been proposed. For example, Patent document 1 proposes highly oil-absorbing calcium carbonate having a BET specific surface area of 25 to 50 m2/g and showing an oil absorption of 120 ml/100 g or higher as determined by the Ogura method (using boiled linseed oil). This calcium carbonate is in the form of linked particles obtainable by performing carbonation reaction at a starting temperature of 17 to 38° C. using a calcium hydroxide suspension containing a chelating agent etc.
The applicant of this application also proposed light calcium carbonate obtained by floc aggregation of primary particles having a large aspect ratio, which has a BET specific surface area of 8 to 20 m2/g and a pore volume of 1.5 to 3.5 cm3/g, as calcium carbonate suitable for internal loading used in papermaking (Patent document 2). This calcium carbonate enables production of bulky internally filler-loaded paper when it is used as a filler, because it has a pore volume in the specific range.    Patent document 1: Japanese Patent Publication (KOKOKU) No. 5-11051    Patent document 2: International Patent Publication WO2004/108597